The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not admitted to describe or constitute prior art to the present invention.
The efficient regulation of cholesterol biosynthesis, metabolism, acquisition and transport is an essential function of mammalian cells. High levels of cholesterol are associated with atherosclerosis, a leading cause of death in the western world and a major risk factor correlated with the occurrence of coronary heart disease and stroke. Until recently, recommendations for the treatment of hypercholestemia were focused on the use of statins, which inhibit the de novo biosynthesis of cholesterol, and the use of bile acid sequestering agents. While statin-based agents are still in widespread use as cholesterol-lowering drugs, an evolving understanding of the mechanisms controlling cholesterol homeostasis has led to new molecular targets as candidates in therapeutic intervention.
Cholesterol metabolism is controlled through a complex feedback loop involving cholesterol itself and bile acids (which are primary oxidation products), and through secretion in the gut, the single most critical regulators of cholesterol absorption. The nuclear receptors LXR (liver X receptor) and FXR (farnesoid X receptor) are the specialized sensors of cholesterol and bile acids that control transcription of networks encoding key metabolic enzymes. For example activation of LXR by oxysterols (i.e., mono-oxygenated cholesterol metabolites) leads to the up-regulation of CYP7A1, the enzyme that catalyzes the rate limiting step in the conversion of cholesterol to bile acids. In turn, bile acids such as chenodeoxycholic acid (CDCA, 1, a low affinity endogenous agonist for FXR, whose structure is shown below) are potent ligands for FXR, whose activation leads to down-regulation of CYP7A1, leading to the completion of the feedback circuit.
In this circuit FXR induces the expression of a transcriptional repressor SHP (small heterodimer partner) which in turn binds to LRH-1 (liver receptor homolog), which is required in CYP7A activation. Additionally, both LXR and FXR are implicated in the regulation of several other gene products involved in cholesterol absorption, metabolism and transport.
Thus, the identification of potent, selective, small molecule FXR agonists, partial agonists and antagonists would be powerful tools and would have many potential applications. For example, such compounds would facilitate the in vivo analysis of FXR physiology in vivo. In addition, such compounds, in conjunction with DNA arraying technology, might allow for the discovery of new gene products under the control of FXR. Further, FXR modulators might find potential utility in the treatment of cholestasis and other disease states associated with aberrant levels, flow and release of bile acids. Moreover, in the absence of a crystal structure of FXR, a thorough structure-activity relationship (SAR) study of ligands that modulate the activity of FXR would allow for the delineation of the structural requirements for ligand binding and might aid in the design of future ligands and potential therapeutics.